This invention relates to a fluid suspension system for a motor vehicle, and in particular to a motor vehicle air suspension system.
Traditionally motor vehicle suspension systems utilise leaf springs associated with the wheels of the vehicle. Such systems are generally satisfactory, in that they adequately control brake and drive torques, as well as preventing lateral movement of the vehicle axles relative to the vehicle chassis. They suffer, however, from the disadvantages of a relatively poor ride, and a relatively large degree of damage to roads.
Air suspension systems are known, and these are generally superior to traditional leaf spring systems in that there is a substantial reduction in road damage, and a softer ride (which gives rise to increased passenger comfort). Unfortunately, known air suspension systems cannot adequately control the lateral movement of the associated axles. Where such movement occurs, there is a tendency for the associated wheels to hit the vehicle bodywork or associated components (such as the air bellows of the suspension system), and this is obviously undesirable. There is also a tendency for axle tracking to occur, and this could lead to the vehicle crabbing badly (that is to say moving in a skew fashion rather than in a straight line). Here, again, this is clearly undesirable.
For example, a known type of air suspension system has an air bellows mounted between a load arm and the chassis of the vehicle. One end of the load arm is pivotally mounted to the chassis forwardly of the wheel being suspended, the other end of the load arm being fixed to the associated axle seat of the axle carrying that wheel. Because of its configuration, this known type of system is known as a trailing arm air suspension system. This system suffers badly from lateral axle movement. It also cannot adequately control the forward and rearward axle tilt resulting from brake and drive torque, unless the load arm is very strong and stiff. If this is the case, however, there is no play in the system, and brake and drive torques are transmitted to the axle casing, and this can lead to breakage to the axle casing. This system can also result in breakage of the axle casing where the axle tilts excessively about the central longitudinal axis of the vehicle, which could arise, for example, from excessively tight cornering or from one wheel hitting an obstacle such as a bump or a pot-hole.
Another known type of air suspension system (see DEAS No. 2224315) utilises a leaf spring or a torsion bar stabiliser for limiting lateral axle movement. This system has the fluid suspension device behind the axle, the fluid suspension device being supported by a load arm which is pivotally attached to the chassis well forward of the axle. Because of the long moment arm between the articulation point of the load arm and the fluid suspension device, this device is pressurised to a significantly less extent than is the case with the standard trailing arm system referred to above. As a result, this type of suspension leads to an excessively soft or "sickly" ride, which is disadvantageous. Another disadvantage of this system is that it does not permit the axle to oscillate about the central longitudinal axis of the vehicle. Consequently, if one wheel only encounters an obstacle such as a bump or pot-hole (or during excessively tight cornering), the entire system can jam with the chassis at an angle to the road surface. Clearly, this will adversely affect the steering and handling of the vehicle, and so is undesirable.
The aim of the invention is to provide a motor vehicle air suspension system which substantially prevents lateral axle movement, and which permits control of brake and drive torques, whilst having sufficient play to prevent excessive forces being transferred to the axle casing.